Paper materials, plastic film materials, and other materials commonly provided in sheet and strip form are often initially produced as continuous lengths of material, usually referred to as “webs,” which are wound onto cores. As used herein, the term “web” is intended to refer both to such continuous lengths of material and to shorter and narrower, so called “sheets” of material. During manufacture, the web can stretch or shrink unevenly across the cross-web (CD) direction and in the machine direction (MD) of the web. When the web returns to its steady-state condition, the stretched or shrunken areas will often become baggy, which can develop into creases or wrinkles in the web. These changes in the condition of the web can cause problems in later web handling operations, including conveying through nips, slitting and winding. For example, it is more difficult to properly slit a baggy web into strips than to slit a web without bagginess.
According to David R. Roisum, in his 2002 PFFC Peer-Reviewed Paper entitled “BAGGY WEBS: MAKING, MEASUREMENT AND MITIGATION THEREOF,” bagginess can be defined as a deviation of flatness. That is, web that refuses to lay or run flat and straight is said to be baggy. Depending on the particular industry, bagginess is also related to terms such as “baggy lanes”, “camber”, “frogbellies”, “layflat”, “puckers”, and “non-planarity”.
Bagginess in a web poses several difficulties including poor visual appearance, defects during coating or lamination, impediments to floating over rollers, differences in winding tightness, and problems in web guiding, tracking and path control. A web having excessive bagginess may refuse to go through nips.
Bagginess is very difficult to measure in a quantifiable way. Common instruments and measurements do not correlate well to bagginess because they do not measure anything closely related to it. While there are ways to measure bagginess more directly, most are tedious or fraught with uncertainty or both. Thus, culling and rejection is typically done by subjective visual inspection.
One test for bagginess involves laying a web on an inspection table whereby the web is under no external stresses that can disturb flatness. A baggy web will not lay flat and/or the edges will not be straight. Roisum suggests that the most important or effective tool for bagginess detection is the eye of the operator. Baggy edges will appear as ruffles. A baggy lane will appear as stitches or “tractor tire” marks oriented in the machine direction. However, Roisum recognizes that a web can appear to be baggy when it is not because hard wrinkles and curl can cause the web to not lay dead flat even if it is not inherently baggy. Thus, a more rigorous test for inherent bagginess is needed.
Another method of monitoring bagginess of a web is disclosed in U.S. Pat. No. 5,778,724, which issued to T. Clapp et al. on Jul. 14, 1998. In the Clapp et al. method, a first reference light is projected onto a front face of the web transverse to the web, and a first measurement light is projected onto the front face of the web non-perpendicular to the front face and transverse to the web. The longitudinal distance on the front face of the web between a point along the first reference light and a corresponding point along the first measurement light are compared to determine bagginess of the web. In the Clapp et al. process, two laser line sources are required: one that is normal to the web in the cross-direction and another that grazes the web in the cross-direction. Relative distance between the lines indicates extent of bagginess. This is primarily an “on-line” tool. Only a single line of cross-direction web bagginess information is collected at a time. The method does not attempt to reconstruct the three-dimensional surface, as very simple two-dimensional geometry math is used. Clapp et al. make no reference to a need to control web tension. For such an on-line system, the minimum amount of tension that is needed to convey the web may indeed be too high to expose problematic bagginess (i.e., non-planarity). Even minor amounts of non-planarity can cause severe problems in subsequent finishing operations requiring conveyance of adjacent web slits through nip rollers and crosscut sheeters.
Accordingly, there exists a need to control web tension at a level much lower than the typical paper manufacturing process in order to expose and measure small degrees of non-planarity that are not visible at higher tensions. There is also a need to use multiple lines, in order to gather significantly more information about true non-planarity, and analyze and reconstruct the entire surface topology of a 2D section of web.